1. Field of the Invention
The present invention relates to an image pickup apparatus using zoom lens.
2. Description of the Related Art
An image pickup apparatus using zoom lens has hitherto been known. A method of controlling an exposure time and an F number of an aperture stop has been widely used as a method for adjusting an exposure at the time of capturing of one frame of the image pickup apparatus.
Moreover, as a method which is widely used for a video application, a method of adjusting by insertion and withdrawal of a light-amount reducing filter in an optical path of a zoom lens has been known from Japanese Patent Application Laid-open Publication No. 2006-267863.
By using such method, it is easy to achieve an appropriate exposure without making an entrance-pupil diameter of the zoom lens excessively small, and to achieve and image with a small depth of field even in a bright environment (an image of an abstract description).
Moreover, even when focal-point detection is any of a TTL phase-difference detection method or a TTL contrast detection method, a predetermined entrance-pupil diameter has to be secured, and even at the time of video photography in the bright environment, the focal-point detection can be carried out with the entrance-pupil diameter secured, and there is a merit particularly at the time of video photography.
In Japanese Patent Application Laid-open Publication No. 2006-267863, a zoom lens in which, the aperture stop is disposed immediately before an object side of a lens unit of a positive refractive power, and the light-amount reducing filter (ND filter) which can be inserted in and withdrawn from is disposed immediately after an image side of the lens unit having positive refractive power, has been disclosed. When the light-amount reducing filter is disposed immediately after the object side of the lens unit having a positive refractive power together with the aperture stop, it becomes difficult to bring a positive lens unit at a telephoto-end close to the lens unit immediately before the object side. However, taking into consideration the abovementioned point, an arrangement which is advantageous for securing a zoom ratio is adapted in a structure of Japanese Patent Application Laid-open Publication No. 2006-267863.